With the trend toward fineness in LSI (large scale integrated) circuit processing, resist materials for lithography are also required to have high resolution capabilities. From the standpoint of forming especially fine patterns, photolithography using short wavelength light, a lens with a large numerical aperture, or lithography using high energy rays such as electron rays, are regarded as advantageous. However, use of such a lithography illuminant or optical system for attaining higher resolution capabilities causes the photolithography to have a reduced focal depth and the high energy ray lithography to have problems such as vicinal effects, etc. As an expedient to solve such a problem, it has been proposed to conduct lithography using a multilayered resist coating. In particular, in practical use of a multilayered resist, a pattern formed in the upper layer by lithography is transferred to the lower layer by reactive ion etching with an oxygen plasma treatment (hereinafter abbreviated as O.sub.2 -RIE) with the patterned upper layer as a mask, thereby obtaining a resist pattern having a high aspect ratio (B. J. Lin, Solid State Technol., 24, 73 (1981)). In the above method, the upper layer of the resist is particularly required to have high resistance to O.sub.2 -RIE. For this purpose, it has been proposed to form a silicon-containing polymer layer, as an inner mask, between the upper and lower layer, to make up for the insufficient O.sub.2 -RIE resistance of the upper layer. However, it is more preferred to enhance the O.sub.2 -RIE resistance by incorporating silicon into the upper layer of the resist.
For such a purpose, many resist material techniques have been proposed. The use of an alkali-soluble polysilsesquioxane (hereinafter abbreviated as APSQ) containing an acetyl group and/or hydroxyl group as the main polymer ingredient is one of the most advantageous resist materials for practical use. Such resists are described, for example, in JP-A-63-239440 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"), JP-A-63-269150, JP-A-1-201337, JP-A-1-201653, JP-A-64-44933, JP-A-64-46746, JP-A-64-46747, and JP-A-1-222254. As described in these references, APSQ can be used, along with a naphthoquinonediazide compound, to form a positive photoresist, from which a fine image can be formed through alkali development in a manner similar to ordinary positive photoresists. The APSQ-based composition may also be used as a material for forming a negative resist pattern using an image-reversal technique. It can further be used to form an image by means of high energy rays. In addition to circuit pattern formation, it is usable for other applications such as an interlaminar insulating film. The APSQ is characterized as having exceedingly high heat resistance because of its high glass transition temperature.
However, all of the resists employing APSQ have a common drawback, in that an insoluble residue (scum) is apt to form in areas to be removed by development after light exposure. Scum is formed in those areas of an extracted pattern which are to be removed by development in the case of positive image formation. In the case of using an image-reversal technique, scum appears in those areas of a pattern which are to be removed by a second exposure to light.
Formation of such scum in circuit patterns is not desirable, in that it may cause wiring omissions or short circuits in semiconductor circuits obtained through processing using these resist patterns. The same scum phenomenon is also observed with ordinary (silicon-omitting) positive photoresists consisting of a novolak resin and a naphthoquinonediazide photosensitive substance. For these ordinary positive photoresists, it has been disclosed, for example, in JP-A-63-25650, to prevent scum formation by adding an additive to developing solutions and by selecting the optimum composition and structure for the novolak resin (disclosed in, for example, JP-A-63-2044 and JP-A-64-11259). However, a general explanation for the cause of scum formation has not yet been established. In the case of silicon-containing resists comprising different base polymers, there has been no understanding as to which factors bring about scum. Because of the above, it is difficult to eliminate the scum problem from the silicon-containing resists utilizing APSQ and, hence, there has been a strong desire for development of a resist that is free from such problem.